Transplastomic

Transplastomic refers to organisms whose plastid genomes have been genetically modified to carry foreign genes. In plants, this typically means engineering the chloroplast genome, though similar approaches have been explored in other plastids. The engineering is done by introducing a DNA construct into chloroplasts and selecting for transformants via homologous recombination into the plastid DNA. The construct includes flanking sequences matching regions of the plastid genome, a gene of interest, and regulatory elements. Transformation methods include biolistic delivery and, in some systems, PEG-mediated protoplast transformation. Expression is driven by plastid promoters and UTRs, enabling high levels of protein accumulation. In many constructs, medicinal or industrial genes are placed in operon-like arrangements, and a selectable marker such as aadA confers resistance to spectinomycin.

Plastid transformations have several notable advantages. The plastid genome exists in high copy number per cell, permitting high expression levels; plastids can express multiple genes from a single promoter; transgenes can be contained within the plant because chloroplast inheritance is typically maternal, reducing pollen-mediated gene flow. These features have led to work in producing pharmaceutical proteins, enzymes, and nutritionally enhanced crops.

Limitations and considerations include the fact that not all species are readily transformable; editing plastid genomes can be technically challenging; plastids have limited post-translational modification capabilities; potential regulatory and public acceptance issues; and maternal inheritance is not universal, with occasional paternal leakage observed in some species. Regulatory frameworks vary by country and trait.